Part 8
+-------------+------------------+------------------+-------------------+ |=Life Ages | =Rock Systems= | =Series of | =Forms of Life= | |of the Earth=| | Rock Strata= | | +-------------+------------------+------------------+-------------------+ | | |Recent, or Human. |Man predominant. | | | | | | | | |=Pleistocene= |Mammoth, mastodon, | | | |(_plīs´tŏ-sēn_), |bear, bison, rein- | | |=Quaternary= |or “most recent.” |deer, musk-ox. | | |(_kwa-ter´na-ri_) |Glacial Period. |Possibly man was | | |or “fourth.” Once | |living but that is | | |supposed to be the| |uncertain. | | |_fourth_ sedimen- | | | | |tary system. Age |=Pliocene= |Plants and animals | | |of man. |(_plī´ō-sēn_), or |much as today, | | | |“more recent.” |aside from human | | | | |and domestic | | | | |species. | | +------------------+------------------+-------------------+ |=Cenozoic= | |=Miocene= (_mī´ō- |Land animals in- | |(_se´nō-zō´- | |sēn_), or “less |clude elephants, | |ik_), or | |recent.” |camels, deer, oxen,| |“Recent | | |horses, true apes, | |life.” | | |etc. Marine animals| | | | |much like those to-| |_Estimated | | |day. Among plants, | |Age of | | |grasses become im- | |Period_, | | |portant; deciduous | |=3,000,000= | | |trees increase. | |_years_. |=Tertiary= (_ter´-| | | | |shi-a-ri_), or |=Oligocene= |Ancient dogs, cats,| | |“third”. Once |(_ōl´ĕ-gō-sēn_), |rabbits, squirrels,| | |supposed to be the|or “a little more |camels, and horses | | |_third_ sedimen- |recent.” |were represented. | | |tary system, or | | | | |Age of mammals. | | | | | |=Eocene= (_ē´-ō- |Mammals flourished,| | | |sēn_), or “dawn of|including rodentia,| | | |recent.” |carnivera, eden- | | | | |tates, lemuroids, | | | | |birds, reptiles, | | | | |etc. Flora included| | | | |figs, palms, | | | | |bananas; willows, | | | | |chestnuts, oaks, | | | | |etc. | +-------------+------------------+------------------+-------------------+ | | |{Upper. |Reptiles predomi- | | | |{ |nate: turtles, | | | |{ |lizards, | | | |{ |crocodiles, flying | | | |{ |reptiles, etc. Many| | | |{ |waterbirds. Angio- | | |=Cretaceous= |{ |sperms predominate:| | |(_krē-ta´-she-us_)|{ |larch, beech, | | |or “bearing |{ |walnut, tulip | | |chalk.” |{ |trees, etc. | | | |{ | | |=Mesozoic= | |{Lower. |Reptiles abound. | |(_mĕs-ō-zō´- | |{ |Flora includes | |ic_), or | |{ |cycadeous, coni- | |“Middle | |{ |fers, horsetails; | |Life.” | |{ |angiosperms appear.| | +------------------+------------------+-------------------+ |_Estimated | |{Upper. |Ammonites, belem- | |Age of |=Jurassic= (_jȯȯ- |{ |ites continue in | |Period_, |ras´sik_), or like|{ |great variety. | |=5,000,000= |the mass of the |{Middle. |Reptiles numerous | |_years_. |Jura Mountains. |{ |and varied types. | | |Age of Reptiles. |{ |Flying reptiles and| | | |{Lower. |reptile-like birds | | | |{ |appear. | | | | | | | | |{Upper. |Reptiles of enor- | | |=Triassic= |{ |mous size dominate | | |(_trĭ-ăs´ĭk_), or |{ |the land and sea. | | |in a triple |{Middle. |Mammals appear. | | |series. |{ |Ammonites and | | | |{ |belemites dominate | | | |{Lower. |invertebrate life. | +-------------+------------------+------------------+-------------------+ | | |=Permian= (_per´- |Reptiles become | | | |mē-ăn_), like |prominent in number| | | |those at Perm, |and variety; in- | | | |Russia. |habit fresh water, | | | | |salt water and | | | | |land. | | | | | | | |=Carboniferous= |=Pennsylvanian=, |Plants abound. | | |(_kăr-bŏn-if´-er- |like those of |Marked development | | |us_), or coal- |Pennsylvania. |of land animals, | | |bearing. Age of | |including insects, | | |Amphibians. | |spiders and scorpi-| | | | |ons. Lizards become| | | | |important. Amphibi-| | | | |ans reach climax. | | | | | | | | |=Mississippian=, |Crinoids greatly | | | |or Lower Carboni- |developed. Am- | | | |ferous. |phibians appear. | | | | |Plant life expands.| | +------------------+------------------+-------------------+ | | |{Upper. |Rapid changes in | | | |{ |animal kingdom; | | |=Devonian= (_de- |{ |shifting habitat; | | |vō´ni-an_) like |{ |extensive develop- | | |those of Devon- |{Middle. |ment of fishes; | |=Paleozoic= |shire, England. |{ |sharks flourish. | |(_pāl-æ-ô- |Age of Fishes. |{ |Plants are mainly | |zō´ic_), or | |{ |small leaf and reed| | “Old Life.” | |{Lower. |types. | | +------------------+------------------+-------------------+ |_Estimated | |{=Ontarian= (_on- |Vertebrates appear;| |Age of |=Silurian= (_si- |{tā´rē-ăn_), place|low forms of | |Period_, |lū´ri-an_), in the|{name. |fishes. First reef | |=24,000,000= |land of the |{ |building corals. | |_years_. |Silures, England. |{=Champlainian= |Crinoids and bra- | | |Age of In- |{(_shăm-plān´ē- |chiopods, important| | |vertebrates. |{ăn_), place name.|Cephalopods con- | | | |{ |tinue to dominate. | | +------------------+------------------+-------------------+ | | |{=Cincinnatian= |Much as in the Cam-| | | |{(_sĭn-sĭn-năt´-ē-|brian. Remains are | | | |{ăn_), place name.|more abundant. | | |=Ordovician= (_ŏr-|{ |Species more numer-| | |dŏ-vīsh´ăn_), a |{=Mohawkian= (_mō-|ous; insects were | | |place name in |{hŏk´ē-ăn_), place|present. Verte- | | |Wales. |{name. |brates appear. Low | | | |{ |forms of fishes. | | | |{ |Trilobites reach | | | |{ |climax. | | +------------------+------------------+-------------------+ | | |{=Saratogan= |All great divisions| | | |{(_săr-ă-tō´găn_),|of animal kingdom | | | |{place name. |except vertebrates | | |=Cambrian= (_kam´-|{ |are represented; | | |bri-an_), from |{=Acadian= (_ä- |trilobites, bra- | | |Cambria, the old |{kād´ē-ăn_), place|chiopods, sponges, | | |name for Wales. |{name. |graptolites, etc. | | | |{ |Little evidence of | | | |{=Georgian= (_jōr´|vegetation, but it | | | |{gē-ăn_), place |must have abounded | | | |{name. |as food for | | | |{ |animals. | +-------------+------------------+------------------+-------------------+ |=Proterozoic=| |{=Keweenawan=, |Fossils rare or | |(_prō-ter-ō- | |{(_kē´wē-năh- |wanting. | |zō´ik_) or |=Algonkian= (_ăl- |{wān_), pertaining| | |“Former |gŏn´kē-ăn_), from |{to Keweenaw Pen- | | |Life.” |district of |{insula, Michigan.| | | |Algonquin |{ | | |_Estimated |Indians, north of |{=Huronian= (_hu- |Rocks contain clear| |Age of |St. Lawrence. |{rō´nē-ăn,_), |evidence of low | |Period_, | |{rocks on borders |forms of life. | |=18,000,000= | |{of Lake Huron. | | |_years_. | |{ | | +-------------+------------------+------------------+-------------------+ | | |{=Laurentian= |Since the rocks are| |=Archaeozoic=| |{(_law-ren´shi- |of igneous origin, | |(_ar´kē-o-zō´| |{an_), pertaining |they contain no | |ic_), “With- | |{to rocks along |organic remains. | |out Life.” | |{the St. Lawrence | | | |=Archean= (_är- |{River. | | |_Estimated |kē´-ăn_), |{ | | |Age of |“oldest.” |{=Keewatin= (_kē- |No fossils found, | |Period_, | |{wā´tĭn_), rocks |but carbonaceous | |=18,000,000= | |{in a district of |schists and lime- | |_years_. | |{Manitoba, Canada.|stones are believed| | | |{ |to indicate the | | | |{ |presence of life. | +-------------+------------------+------------------+-------------------+
+-------------+------------------+------------------+-------------------+ |=Life Ages | =Rock Systems= | =Series of | =Chief Economic | |of the Earth=| | Rock Strata= | Products= | +-------------+------------------+------------------+-------------------+ | | |Recent, or Human. |Clay, peat, bog | | | | |iron ore, marl, | | | | |gold placers. | | |=Quaternary= | | | | |(_kwa-ter´na-ri_) |=Pleistocene= |Clay, gravel, | | |or “fourth.” Once |(_plīs´tŏ-sēn_), |gold placers. | | |supposed to be the|or “most recent.” | | |=Cenozoic= |_fourth_ sedimen- |Glacial Period. | | |(_se´nō-zō´- |tary system. Age | | | |ik_), or |of man. |=Pliocene= |Gold (in part | |“Recent | |(_plī´ō-sēn_), or |placers), coal, | |life.” | |“more recent.” |oil, gas. | | +------------------+------------------+-------------------+ |_Estimated | |=Miocene= (_mī´ō- |Silver, gold, coal,| |Age of | |sēn_), or “less |oil, gas, phosphate| |Period_, | |recent.” |rock, diatomaceous | |=3,000,000= |=Tertiary= (_ter´-| |earth. | |_years_. |shi-a-ri_), or | | | | |“third”. Once |=Oligocene= |Copper, silver. | | |supposed to be the|(_ōl´ĕ-gō-sēn_), | | | |_third_ sedimen- |or “a little more | | | |tary system, or |recent.” | | | |Age of mammals. | | | | | |=Eocene= (_ē´-ō- |Gold, zinc, lead, | | | |sēn_), or “dawn of|coal, oil, gas. | | | |recent.” | | +-------------+------------------+------------------+-------------------+ | |=Cretaceous= |{ |Coal, oil, gas, | | |(_krē-ta´-she-us_)|{Upper. |copper, gold, china| | |or “bearing |{ |clay, fire clay, | | |chalk.” |{Lower. |cement building | |=Mesozoic= | |{ |stone. | |(_mĕs-ō-zō´- | |{ | | |ic_), or | |{ | | |“Middle +------------------+------------------+-------------------+ |Life.” |=Jurassic= (_jȯȯ- |{Upper. |Oil, gold. | | |ras´sik_), or like|{ | | |_Estimated |the mass of the |{Middle. | | |Age of |Jura Mountains. |{ | | |Period_, |Age of Reptiles. |{Lower. | | |=5,000,000= | | | | |_years_. | |{Upper. |Salt, gypsum, a | | |=Triassic= |{ |little coal in | | |(_trĭ-ăs´ĭk_), or |{Middle. |Virginia, copper, | | |in a triple |{ |building stone. | | |series. |{Lower. | | +-------------+------------------+------------------+-------------------+ | | |=Permian= (_per´- |Salt and gypsum; | | | |mē-ăn_), like |some coal in | | | |those at Perm, |Eastern States. | | | |Russia. | | | | | | | | |=Carboniferous= |=Pennsylvanian=, |Coal, oil, gas, | | |(_kăr-bŏn-if´-er- |like those of |iron ore, fire | | |us_), or coal- |Pennsylvania. |clay, phosphate | | |bearing. Age of | |rock. | | |Amphibians. | | | | | | | | | | |=Mississippian=, |Oil, gas, lead, | | | |or Lower Carboni- |zinc, building | | | |ferous. |stone, cement rock.| | +------------------+------------------+-------------------+ | |=Devonian= (_de- |{Upper. | | | |vō´ni-an_) like |{ |Gas, oil, iron ore,| | |those of Devon- |{Middle. |phosphate rock. | |=Paleozoic= |shire, England. |{ | | |(_pāl-œ-ô- |Age of Fishes. |{Lower. | | |zō´ic_), or +------------------+------------------+-------------------+ | “Old Life.” | |{=Ontarian= (_on- |Iron ore, gas, | | |=Silurian= (_si- |{tā´rē-ăn_), place|salt, gypsum, | |_Estimated |lū´ri-an_), in the|{name. |cement rock. | |Age of |land of the |{ | | |Period_, |Silures, England. |{=Champlainian= | | |=24,000,000= |Age of In- |{(_shăm-plān´ē- | | |_years_. |vertebrates. |{ăn_), place name.| | | +------------------+------------------+-------------------+ | | |{=Cincinnatian= |Oil, gas, lead, | | |=Ordovician= (_ŏr-|{(_sĭn-sĭn-năt´-ē-|zinc, phosphate | | |dŏ-vīsh´ăn_), a |{ăn_), place name.|rock, manganese, | | |place name in |{ |marble. | | |Wales. |{=Mohawkian= (_mō-| | | | |{hŏk´ē-ăn_), place| | | | |{name. | | | +------------------+------------------+-------------------+ | | |{=Saratogan= |Lead, zinc, barite,| | | |{(_săr-ă-tō´găn_),|copper. | | | |{place name. | | | |=Cambrian= (_kam´-|{ | | | |bri-an_) from |{=Acadian= (_ä- | | | |Cambria, the old |{kād´ē-ăn_), place| | | |name for Wales. |{name. | | | | |{ | | | | |{=Georgian= (_jōr´| | | | |{gē-ăn_), place | | | | |{name. | | +-------------+------------------+------------------+-------------------+ | | |{=Keweenawan=, |Copper, silver. | |=Proterozoic=| |{(_kē´wē-năh- | | |(_prō-ter-ō- | |{wān_), pertaining| | |zō´ik_) or | |{to Keweenaw Pen- | | |“Former |=Algonkian= (_ăl- |{insula, Michigan.| | |Life.” |gŏn´kē-ăn_), from |{ | | | |district of |{=Huronian= (_hu- |Principal iron ores| |_Estimated |Algonquin |{rō´nē-ăn,_), |of Lake Superior | |Age of |Indians, north of |{rocks on borders |region; also | |Period_, |St. Lawrence. |{of Lake Huron. |copper, nickel, | |=18,000,000= | |{ |silver, cobalt, | |_years_. | |{ |gold. Building | | | |{ |stone and ornamen- | | | |{ |tal stone. | +-------------+------------------+------------------+-------------------+ | | |{=Laurentian= |Iron ores, precious| |=Archaeozoic=| |{(_law-ren´shi- |metals, gems, | |(_ar´kē-o-zō´| |{an_), pertaining |apatite, rare | |ic_), “With- | |{to rocks along |earths, graphite, | |out Life.” | |{the St. Lawrence |asbestos. | | |=Archean= (_är- |{River. | | |_Estimated |kē´-ăn_), |{ | | |Age of |“oldest.” |{=Keewatin= (_kē- |Emery, building and| |Period_, | |{wā´tĭn_), rocks |ornamental stones. | |=18,000,000= | |{in a district of | | |_years_. | |{Manitoba, Canada.| | +-------------+------------------+------------------+-------------------+
[Illustration: =GEOLOGICAL MAP OF THE UNITED STATES SHOWING THE REGIONS OF REPRESENTATIVE FORMATIONS=]
THE SURFACE OF THE EARTH
[Illustration: =MAP SHOWING THE DISTRIBUTION OF LAND AND WATER=]
LAND FORMS OF THE WORLD
The proportion of land to water upon the earth is as 27 to 72, or roughly _one-fourth_ to _three-fourths_; the land covering fifty-three million square miles, the sea one hundred and forty-four million. The land consists of six great bodies called continents, and a multitude of small fragments called islands, which skirt the shores of the continents or dot the broad expanse of the sea.
THE DISTRIBUTION OF LAND AND WATER
By far the greatest proportion of land is in the northern hemisphere, and in temperate latitudes. Broadly speaking, the northern hemisphere is the hemisphere of land, and the southern hemisphere is the hemisphere of ocean. The earth could be bisected in such a way that one hemisphere contained almost no land, while the other was composed almost equally of land and water.
LOCATION OF THE CONTINENTS
The greater part of the land on the earth’s surface is grouped into two great _hemispheres_, the Old and the New World. The former and far larger of these consists of Eurasia in the north, separated by ill-defined boundaries from Europe to the west and Asia to the east, and of Africa in the south, united to Eurasia by the narrow neck of the isthmus of Suez. The hemisphere of the New World is divided into North America and South America, united by the long, narrow isthmus of Central America. The island of Australia is also reckoned as a continent. It is believed that an island continent, Antarctica, surrounds the South Pole. Of islands not reckoned as continents, the largest is the polar island of Greenland.
CERTAIN RESEMBLANCES OF THE CONTINENTS
In comparing the continents, we at once notice certain resemblances. The first is the tapering to the south, which is seen in Greenland, North and South America, Africa, and Australia (Tasmania). Another is the southward-running peninsulas which characterize Europe and Asia. We may notice, too, that the general lines of the Old World, broad in the north, tapering in the south, resemble those of the New World, especially if we include Australia (Tasmania), and compare its position with that of South America. There is also a certain uniformity in the distribution of relief. Notice the so-called Mid-World and Pacific Mountain systems, which may be traced in the mountains of Central Europe, North Africa, Central Asia, the islands of the Pacific from Japan to New Guinea, and the lofty mountains of North, Central, and South America.
[Illustration: =DIAGRAM SHOWING AVERAGE HEIGHT OF THE CONTINENTS=]
COMPARISON OF THE CONTINENTS
+---------------------+------+------+------+------+------+-----+------+ | |=Asia=|=Afri-|=North|=South| =Eu- |=Aus-| =All | | =Continent= | | ca= |Ameri-|Ameri-|rope= |tra- |Land= | | | | | ca= | ca= | |lia= | | +---------------------+------+------+------+------+------+-----+------+ |Area (million square | | | | | | | | |miles) | 16.4| 11.1| 7.6| 6.8| 3.7| 3.0| 55.0| |Average Height (feet)| 3,000| 2,500| 1,900| 2,000| 940| 800| 2,100| |Highest Point (feet) |29,000|18,800|18,200|22,400|18,500|7,200|29,000| | | | | | | | | | |PERCENTAGE AT VARIOUS| | | | | | | | | ALTITUDES (feet) | | | | | | | | | | | | | | | | | |Below Sea-Level | 1.4 | 0.1 | 0.05| 0.0 | 1.8 | 0.0| 0.6| | 0 to 600 feet| 23.3 | 12.5 | 32.25| 40.0 | 53.8 | 29.8| 26.7| | 600 to 1,500 feet| 16.0 | 34.8 | 32.1 | 26.8 | 27.0 | 64.3| 27.8| | 1,500 to 3,000 feet| 21.7 | 27.6 | 13.3 | 16.8 | 10.0 | 4.1| 19.3| | 3,000 to 6,000 feet| 21.8 | 21.8 | 13.2 | 7.0 | 5.5 | 1.5| 17.0| | 6,000 to 12,000 feet| 10.0 | 2.8 | 8.4 | 5.0 | 1.7 | 0.3| 6.0| | Above 12,000 feet | 5.8 | 0.4 | 0.7 | 4.4 | 0.2 | 0.0| 2.6| +---------------------+------+------+------+------+------+-----+------+
THE SHAPING OF THE COAST
The coast line, or margin of sea and land, is an area rapidly wearing away under the ceaseless influence of the waves, and of the sand and rock, they are perpetually hurling to and fro. Coasts may be either flat or high, composed either of hard or soft rock, and either submerged or raised. A submerged coast is one where the land has sunk or the sea has risen, so that the low grounds and valleys are flooded. A raised coast is one where the land has risen or the sea has retired, and what was formerly the sea bottom is bared.
A flat coast is usually sandy, often bordered by sandhills and lagoons. It may be carved into cliffs, as in the clay cliffs of Norfolk, England. A raised coast is usually flat from the long-continued action of the waves during the period when it was submerged. Flat coasts have no good harbors.
A submerged coast differs according to the nature of the submerged region. If this was hilly or mountainous, with valleys running parallel to the shore, the coast will be ironbound and harbor-less unless the sea-level has risen sufficiently to give access to the valleys behind the first range of heights. If this happens, T-shaped gulfs are formed. Where the valleys open at right angles to the sea, they become bays, usually with excellent harbors. The hills between the valleys rise as peninsulas, or islands. If the land was flat before submerging took place, a flat coast is the result.
Where the land is composed of soft rocks, a more uniform coast-line results than where it is composed of harder rocks, or of hard and soft rocks mixed. The waves, in eating out the softer rocks, often form magnificent sea-caves, natural arches, and pinnacles.
THE COASTLINE OF THE VARIOUS CONTINENTS
EUROPE surpasses all the other continents in the magnitude of its indentations and projections. Three great peninsulas--the Balkan peninsula, Italy, and Spain, project into the Mediterranean; while Brittany, Denmark, and Scandinavia jut into the shores of the Atlantic. Even the British Isles are scarcely more than a projection of the continent.
ASIA is a second in the relative extent of its peninsula. Asia Minor on the west, Arabia, India, and Indo-China on the south, and China, Manchuria with Corea and Kamchatka, advancing into the waters of the Pacific, form a wide border of projecting lands, containing the richest regions of the continent.
NORTH AMERICA is considerably less indented. Florida, Nova Scotia and Labrador are more prominent on the Atlantic coast, and California Peninsula and Alaska on the Pacific.
The southern continents on the contrary, are nowhere deeply penetrated by the waters of the ocean. The Gulf of Arica in South America, the Gulf of Guinea in Africa, and the Great Australian Bight, are merely gentle bends in the coast line.
LOCATION OF THE GREAT PLAINS OF THE WORLD
Plains occupy nearly one-half of the surface of the continents. They are most extensive and unbroken on the Arctic slopes of the Old World, and in the interior of the two Americas.
Treeless plains, whose vegetation consists of grasses and other herbaceous plants, or stunted shrubs, occur in every continent, and are designated by a variety of terms. Wherever treeless plains are subject to periodical rains, they lose their verdure in the season of drought, and assume the aspect of a desert; but they resume their freshness on the return of the rain, and many are adorned with a great variety of beautiful flowers.
PLAINS OF THE OLD WORLD. The great Siberian plain extends from the northeastern extremity of Asia to the Ural Mountains and Caspian Sea; and the European plain stretches from the Ural westward, through Russia and North Germany, to the lowlands of Holland.
The plains of the Caspian Sea and western Siberia are dreary steppes, covered with coarse grasses, often growing in tufts, alternating with patches of heather, furze, dwarf birch, and other stunted shrubs; or old sea bottom, covered with salt efflorescence. Immense reaches of flat country, near the Arctic shores of Asia and Europe, consist of frozen marshes, called tundras, where mosses and lichens are almost the only vegetation. Those of eastern Europe and Asia are denominated steppes; while more limited treeless regions in western Europe are called landes and heaths.
On the alluvial plains of the Old World, civilization began and developed; and their inexhaustible fertility supplied the wants of the most populous nations of antiquity. The great centers of ancient civilization in Egypt, China, India and Babylonia, all had their growth in alluvial plains, built up and fertilized by the mighty rivers which traverse those countries.
PLAINS OF THE NEW WORLD. In North America the great _Central Plain_ extends, with but slight interruptions, from the Arctic shores to the Gulf of Mexico. The fertile, treeless plains are termed “prairies” (meadows), while the sterile ones, east of the Rocky Mountains, are known as “the plains.” There are vast cane fields and forests in the lower Mississippi Valley.
In South America the plains of the Orinoco basin, the _Selvas_ of the Amazon, and the _Pampas_ of the La Plata, form an uninterrupted series of lowlands which, continued by the plains of Patagonia to the southern extremity of the continent, extend over a distance of three thousand five hundred miles from north to south. The Spanish term “llano” (plain), and the Peruvian “pampa,” designate the treeless plains of the Orinoco and La Plata basins. The Llanos of the Orinoco, during one-half of the year are covered by the richest pasturage, bright with flowers, but during the other half are a parched waste. The Selvas of the Amazon, a luxuriant forest, cover more than a million square miles; and the treeless Pampas, with their tall grasses and thickets of clover and thistles, illustrate the endless richness and variety of nature.
Alluvial and marine plains generally have but a slight altitude, while the undulating plains are sometimes considerably elevated. The Mississippi Valley, at St. Louis, one thousand miles from the ocean, is hardly four hundred feet above the sea-level; and the Amazon, at an equal distance from the sea, does not reach two hundred and fifty feet. The marine plains adjacent to the Caspian and Aral seas are still lower, the larger portion being below the sea-level.
SITUATION, ELEVATION AND SOIL OF PLATEAUS
Plateaus are situated either between two lofty mountain chains, which form their margins, or descend by successive terraces to the nearest seas; or they pass, by gradations, from the base of high mountains to the low plains in the interior of the continents.
The Great American Basin, between the Rocky and Sierra Nevada Mountains, and the plateau of Tibet, between the Himalaya and Kuenlun mountains, are examples of the first position; and the table-land of Mexico, of the second. The third is seen in the high plains at the eastern foot of the Rocky Mountains, which descend from an altitude of five thousand or six thousand feet, at the foot of the mountains, to the low plains of the Mississippi basin.
The plateaus most remarkable for their elevation are, Tibet, from ten thousand to eighteen thousand feet above the sea; and the elongated valley-like highlands, from ten thousand to thirteen thousand feet high, between the two chains of the Andes, in South America. East Turkestan and Mongolia, in central Asia; the plateau of Iran, in western Asia; Abyssinia, and the vast plateau which occupies all the southern part of Africa; and the broad table-land which fills the western half of North America with a continuous mass of high land, range in height from four thousand to eight thousand feet.
The great peninsulas of Deccan, Arabia, Asia-Minor and Spain, the central plateau of France, and those of Switzerland, Bavaria, and Transylvania, vary from one thousand to four thousand feet in elevation.
SOIL AND CLIMATE OF PLATEAUS
The nature of the soil and climate of great plateaus is in general such as to render them the least useful portions of the continents. Sahara, with an average altitude of 1,000 feet, and the higher plateaus of Mongolia, Iran and parts of the American Basin, may serve as types.
Their surface consists of hardened sand and rock; of hillocks and plains of loose sand constantly shifting by the wind; and of immense tracts, as in Mongolia, covered with pebbles varying from the size of a walnut, or even less, to a foot in diameter: all indicating the original transporting, grinding and depositing of these materials by water.